This invention relates to an optical fiber for communication, and more particularly, to an optical fiber for communication in which an optical fiber is reinforced by coating a high-molecular material thereon.
Previously used optical fiber for communication have an optical fiber which basically consists of a core and a surrounding cladding each made of a material having different refractive index. The difference of refractive index ensures optical transmission. Such optical fibers are, however, mechanically fragile. To provide structural strength, optical fibers are usually covered with a coating of material having relatively high hardness and flexural modulus, such as nylon, polyvinyl formal (PVF), fluoroethylenepropylene copolymers (FEP), polyetylene (PE) or the like. With respect to techniques for coating optical fibers, reference is made to the article "Loss in coated optical fibers" by K. Ishida et. al., 1975 National Convention of the Japanese Electronic Communication Society, Lecture Papers, Part 4, page 927, March, 1975.
The recent advance in technique for manufacturing optical fibers made it possible to increase the strength of a fiber itself. The function of a coating to reinforce an optical fiber became less important. In other words, coating materials which have lower hardness and flexural modulus than the conventional ones are satisfactory as a reinforcement.
Improvements were also made in the transmission loss of optical fibers. It was found that coating causes to increase the transmission loss of optical fibers. The higher the hardness and flexural modulus of a coating material are, the more the transmission loss are increased. Provided that the same coating material is used, the transmission loss are more increased as the temperature is lower. With such increase of transmission loss, repeater spacing is shortened and signal-to-noise ratio is increased so that severer requirements are imposed on the design of peripheral equipment. Various problems are encountered in practice. Generally, it is necessary to maintain transmission loss to at most about 5 dB per kilometer and an increase in transmission loss resulting from the temperature variation from 20.degree. C. to -50.degree. C. to about 2 dB per kilometer to prevent undesired problems from occurring.
The inventors have made an analysis on the above problems.
The reason why the coating of optical fibers increases transmission loss is explained below.
Usually, an optical fiber is provided with a coating by extrusion. The extruded coating shrinks in both the radial and longitudinal directions of the optical fiber when it solidifies.
The radial shrinkage of the coating exerts edgewise pressure to the optical fiber. The interface between the extruded coating and the optical fiber is not always uniform due to the presence of very fine dirt particles attached on the surface of the optical fiber before the coating or foams generated in the coating material as a result of foaming phenomenon. With the interface irregular, the edgewise pressure caused by the radial shrinkage of the coating material may not be uniformly applied to the optical fiber. As a result, the optical fiber is subject to microbending from which light leaks outside.
The longitudinal shrinkage of the coating material exerts shear stress at the interface between the optical fiber and the coating material, which causes to increase the density of a cladding portion of the optical fiber. The refractive index of the cladding portion is correspondingly increased so that the refractive index of the core relative to the cladding in the optical fiber may be varied. This also causes light to leak. It has thus been found that the increase of transmission loss associated with the coating of the optical fiber results from the shrinkage of the coating material upon solidification.
The second is the phenomenon that the transmission loss of an optical fiber are more increased as the hardness and flexural modulus of a coating material are higher. This will be explained as follows.
The coating material which has higher hardness and flexural modulus shows a higher rate of shrinkage, which increases the edgewise pressure to the optical fiber. As a result, microbending loss is increased.
The third is the phenomenon that the coated optical fiber shows an increase in transmission loss as the temperature decreases. This will be apparent from the fact that the rate of shrinkage of the coating material depens on the surrounding temperature.